Primate Locomotion

Primate locomotor habits can be divided into several major categories, each characterized by different patterns of limb use and body positions. These categories are quadrupedal running and climbing, vertical clinging and leaping, arm suspension, and bipedal walking. A primate chiefly uses one of the four types, but may use other types at least some of the time. Primate locomotor characteristics evolved in small quadrupedal mammals with a body weight of less than 100 grams. Contrary to other tree-living small mammals, such as squirrels or tree shrews, primate ancestors inhabited the small branches of the forest canopy, where the diameters of supporting branches are often smaller than the diameter of the animal's trunk. The evolution of prehensile autopodia, combined with the use of a diagonal footfall pattern, provides stability and balance on such unstable supports.

Primates walk along branches with flexed limbs to keep the trunk close to the support. Thus, primates are able to avoid strong vertical oscillations of the trunk by accommodating to uneven substrates. Independent of their anatomical length differences, the fore- and hind limbs undergo equal angular excursions, and their proximal pivots are on the same level. The longer hind limbs are much more flexed than the shorter forelimbs; this allows the animal to accelerate immediately powered by the hind limbs without losing time in flexing the joints.

Arboreal quadrupedalism is the ancestral and most common locomotor mode among primates, and it was the basis for the evolution of specialized locomotor behaviors (for example, slow climbing, terrestrial quadrupedalism, and arm suspension).

Slow quadrupedal walking and climbing is a distinctive mode of quadrupedalism in the Loridae family (lorises and pottos). Members of this group are known for their rather stereotypical, stealthy locō motor behavior, practiced in any direction in space, on the branch or underneath. Loridae do not leap or perform any other fast gait, but they use large step lengths produced by great limb angular excursions. They have a very powerful grasp, because their first digits are more abducted against the other digits. The high oxidative capacity of their limb muscles enables these primates to hold tightly to a branch for hours without fatigue.

[Page 1939]Unlike arboreal quadrupeds, the locomotive apparatus of Old World cercopithecine monkeys shows functional and morphological adaptations to terrestrial running on broad, flat surfaces. These adaptations are seen even in secondary arboreal cercopithecine monkeys. Old World monkeys have a narrow, deep trunk, and parasagittally oriented, more-extended limbs. The fore- and hind-limb length of terrestrial quadruped monkeys is nearly identical.

Primates of all sizes can cross gaps in the three-dimensional meshwork of their arboreal habitat by leaping. The necessary speed for take-off is attained by a rapid extension of the hind limbs from a deep crouched posture. Larger primates gain additional momentum by swinging their long arms. At the end of the leap, primates use their strong, long hind limbs as shock absorbers during the landing.

Vertical clinging and leaping is a highly specialized type of leaping between vertical supports. Vertical clingers and leapers are members of the prosimian families Galagidae, Tarsiidae, Indriidae, and Lepilemuridae. Because they are performed backwards, leaps between vertical supports are more complex than those between horizontal supports. From a clinging-squatted position on a vertical trunk, the thrust of the feet launches the body up in the air, with a rotatory momentum that twists the trunk around its longitudinal axis. The animal rotates to face the target in a symmetrical position. Then, a rotation around the transverse axis shifts the feet to a position suitable for landing. Tarsiers and galagines use their tails to correct their rotatory momentum during the flight phase.

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